1. Technical Field
This disclosure relates to circuits for producing oscillating ramp signals and for producing slope compensation signals in switching regulators.
2. Description of Related Art
Voltage regulators often provide a constant output voltage, regardless of fluctuations in the load or supply voltage.
One type of voltage regulator is a linear voltage regulator. The output voltage is regulated by changing the impedance of a pass element through which current flows from the voltage source to the load. Linear regulators, however, can waste energy and generate substantial heat.
Another type of voltage regulator is a switching voltage regulator. The flow of current from the voltage source to the load is not steady. Instead, it flows in discrete pulses which are usually converted into steady load current by an inductive storage element. By controlling the width of the pulses, the voltage to the load can be regulated.
The width of each pulse may be controlled by an electronic switch, such as a power transistor. The switch may be coupled either in series or in parallel with the load. By controlling the duty cycle of this switch—i.e., the percentage of time that the switch is ON relative to the total period of the switching cycle—a switching voltage regulator can regulate the voltage on the load.
A switching voltage regulator may operate in the current mode, meaning that it may be controlled by a signal indicative of a current in the regulator. However, there may be instability in a current-mode switching voltage regulator when the switching duty cycle exceeds 50%, that is, when the switch is ON for more than 50% of a given switching period. Stability may often still be maintained at such high duty cycles by adjusting the current signal that is used to control the regulator with a slope compensation signal which compensates for the instability.
A portion of an oscillator signal, such as a ramp signal, may often be used as a slope compensation signal. In some applications, however, such as in certain communications circuitry which must operate at high frequencies, the switching voltage regulator must be switched in synchronism with an external clock. In these circuits, instability may still result unless the slope compensation signal is also synchronized to the external clock.
This synchronization may be achieved with a phase locked loop circuit. However, these circuits can be complex, may need large compensation network components, and may take too much time to capture and become synchronized.
Another approach is to add circuitry to detect the presence of the external clock. Once the external clock is detected, the slope compensation may be increased by a fixed factor to account for the maximum synchronizable frequency. One problem with this approach, however, may be overcompensation if the regulator is synchronized just slightly above the normal operating frequency. This may cause the maximum output current to be reduced by the amount of the overcompensation.
Another approach is described in U.S. Pat. No. 6,369,665 to Chee et al., assigned to the assignee of the current invention, and incorporated herein by reference. Chee et al. use a set of digitally controlled current sources, driven by an up-down counter, to provide the needed adjustment. However, this approach can be complex and require a substantial area within a chip on which the circuit is integrated.